A 34-year-old woman presented with epigastric pain, nausea, and dyspepsia. Contrast-enhanced computerized tomography revealed a small mass in the duodenal wall mimicking a periampullary neoplasm and, at endoscopic examination, a periampullary submucosal tumor was suspected. The diagnosis of intramural duodenal diverticulum (IDD) was made by an x-ray barium meal that showed a finger-like sac filled with barium, the so-called "windsock sign." IDD is a rare congenital abnormality caused by an anomalous process of recanalization of the primitive foregut. The intermittent filling and emptying of the IDD is responsible for epigastric pain, nausea, and vomiting. When IDD is symptomatic, surgical or endoscopic treatment is recommended.